Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium based on a mixture of polar compounds having positive dielectric anisotropy, characterized in that it comprises one or more compounds of the general formula I                  
 
in which R, A 1 , Z 1 , Y and L are as defined in claim 1.

The present invention relates to a liquid-crystalline medium, to the usethereof for electro-optical purposes, and to displays containing thismedium.

Liquid crystals are used, in particular, as dielectrics in displaydevices, since the optical properties of such substances can be modifiedby an applied voltage. Electro-optical devices based on liquid crystalsare extremely well known to the person skilled in the art and can bebased on various effects. Examples of such devices are cells havingdynamic scattering, DAP (deformation of aligned phases) cells,guest/host cells, TN (twisted nematic) cells, STN (supertwisted nematic)cells, SBE (superbirefringence effect) cells and OMI (optical modeinterference) cells. The most common display devices are based on theSchadt-Helfrich effect and have a twisted nematic structure.

The liquid-crystal materials must have good chemical and thermalstability and good stability to electric fields and electromagneticradiation. Furthermore, the liquid-crystal materials should haverelatively low viscosity and give short addressing times, low thresholdvoltages and high contrast in the cells.

Furthermore, they should have a suitable mesophase, for example anematic or cholesteric mesophase for the abovementioned cells, atconventional operating temperatures, i.e. in the broadest possible rangeabove and below room temperature. Since liquid crystals are generallyused in the form of mixtures of a plurality of components, it isimportant that the components are readily miscible with one another.Further properties, such as the electrical conductivity, the dielectricanisotropy and the optical anisotropy, must satisfy differentrequirements depending on the cell type and area of application. Forexample, materials for cells having a twisted nematic structure shouldhave positive dielectric anisotropy and low electrical conductivity.

For example, media of large positive dielectric anisotropy, broadnematic phases, relatively low birefringence, very high resistivity,good UV and temperature stability and low vapour pressure are desiredfor matrix liquid-crystal displays having integrated nonlinear elementsfor switching individual pixels (MLC displays).

Matrix liquid-crystal displays of this type are known. Examples ofnonlinear elements which can be used for individual switching ofindividual pixels are active elements (i.e. transistors). This is thenreferred to as an “active matrix”, and a differentiation can be madebetween two types:

-   1. MOS (metal oxide semiconductor) or other diodes on silicon wafers    as substrate.-   2. Thin-film transistors (TFTs) on a glass plate as substrate.

Use of single-crystal silicon as the substrate material limits thedisplay size, since even modular assembly of the various part-displaysresults in problems at the joints.

In the case of the more promising type 2, which is preferred, theelectro-optical effect used is usually the TN effect. A differentiationis made between two technologies: TFTs comprising compoundsemiconductors, such as, for example, CdSe, or TFTs based onpolycrystalline or amorphous silicon. Intensive work is being carriedout worldwide on the latter technology.

The TFT matrix is applied to the inside of one glass plate of thedisplay, whilst the other glass plate carries the transparentcounterelectrode on the inside. Compared with the size of the pixelelectrode, the TFT is very small and has virtually no adverse effect onthe image. This technology can also be extended to fullycolour-compatible image displays, where a mosaic of red, green and bluefilters is arranged in such a way that each filter element is locatedopposite a switchable pixel.

The TFT displays usually operate as TN cells with crossed polarizers intransmission and are illuminated from the back.

The term MLC displays here covers any matrix display containingintegrated nonlinear elements, i.e., in addition to the active matrix,also displays containing passive elements, such as varistors or diodes(MIM=metal-insulator-metal).

MLC displays of this type are particularly suitable for TV applications(for example pocket TV sets) or for high-information displays forcomputer applications (laptops) and in automobile or aircraftconstruction. In addition to problems with respect to the angledependence of the contrast and the response times, problems arise in MLCdisplays owing to inadequate resistivity of the liquid-crystal mixtures[TOGASHI, S., SEKIGUCHI, K., TANABE, H., YAMAMOTO, E., SORIMACHI, K.,TAJIMA, E., WATANABE, H., SHIMIZU, H., Proc. Eurodisplay 84, September1984: A 210–288 Matrix LCD Controlled by Double Stage Diode Rings, p.141 ff, Paris; STROMER, M., Proc. Eurodisplay 84, September 1984: Designof Thin Film Transistors for Matrix Addressing of Television LiquidCrystal Displays, p. 145 ff, Paris]. With decreasing resistance, thecontrast of an MLC display drops, and the problem of after-imageelimination can occur. Since the resistivity of the liquid-crystalmixture generally drops over the life of an MLC display owing tointeraction with the internal surfaces of the display, a high (initial)resistance is very important in order to obtain acceptable servicelives. In particular in the case of low-voltage mixtures, it washitherto not possible to achieve very high resistivities. It isfurthermore important that the resistivity increases as little aspossible with increasing temperature and after heating and/or exposureto UV radiation. Also particularly disadvantageous are thelow-temperature properties of the mixtures from the prior art. It isrequired that crystallization and/or smectic phases do not occur, evenat low temperatures, and that the temperature dependence of theviscosity is as low as possible. MLC displays of the prior art thus donot satisfy current requirements.

There thus continues to be a great demand for MLC displays having veryhigh resistivity at the same time as a broad operating temperaturerange, short response times, even at low temperatures, and low thresholdvoltage which do not have these disadvantages or only do so to a reducedextent.

In the case of TN (Schadt-Helfrich) cells, media are desired whichfacilitate the following advantages in the cells:

-   -   broadened nematic phase range (in particular down to low        temperatures),    -   switchability at extremely low temperatures (outdoor use,        automobiles, avionics),    -   increased stability on exposure to UV radiation (longer life).

The media available from the prior art do not enable these advantages tobe achieved while simultaneously retaining the other parameters.

In the case of supertwisted cells (STN), media are desired which enablegreater multiplexibility and/or lower threshold voltages and/or broadernematic phase ranges (in particular at low temperatures). To this end, afurther extension of the parameter latitude available (clearing point,smectic-nematic transition or melting point, viscosity, dielectricquantities, elastic quantities) is urgently desired.

The invention has the object of providing media, in particular for MLC,TN or STN displays of this type, which do not have the above mentioneddisadvantages, or only do so to a reduced extent, and preferably at thesame time have very high resistivities and low threshold voltages andsimultaneously low values for the rotational viscosity γ₁.

It has now been found that this object can be achieved when novel mediaare used in displays.

The invention thus relates to a liquid-crystalline medium based on amixture of polar compounds having positive dielectric anisotropy,characterized in that it comprises one or more compounds of the generalformula I,

in which

-   R is H, an alkyl or alkenyl radical having 1 to 15 carbon atoms    which is unsubstituted, monosubstituted by CN or CF₃ or at least    monosubstituted by halogen, it also being possible for one or more    CH₂ groups in these radicals to be replaced, in each case    independently of one another, by —O—,

—CO—, —CO—O—, —O—CO— or —S—, —O—CO—O— in such a way that O atoms are notlinked directly to one another,

-   A¹ (a) is a trans-1,4-cyclohexylene radical in which, in addition,    one or more non-adjacent CH₂ groups may have been replaced by —O—    and/or —S—, or a 1,4-cyclohexenylene radical,    -   (b) is a 1,4-phenylene radical, in which, in addition, one or        two CH groups may have been replaced by N,    -   (c) is a radical from the group consisting of        1,4-bicyclo[2.2.2]octylene, piperidine-1,4-diyl,        naphthalene-2,6-diyl, decahydronaphthalene-2,6-diyl and        1,2,3,4-tetrahydronaphthalene-2,6-diyl,    -   where the radicals (a) and (b) may be monosubstituted or        polysubstituted by CN, CH₃ or F,-   Z¹ is —CO—O—, —O—CO—, —CH₂O—, —OCH₂—, —CH₂CH₂—, —CH═CH—, —CF₂O—,    —OCF₂—, —C≡C—, —(CH₂)₄—, —CH═CH—CH₂CH₂— or a single bond,-   Y is F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6    carbon atoms,-   L is H or F, and-   m is 0 or 1

The compounds of the formula I have a broad range of applications.Depending on the choice of substituents, these compounds can serve asbase materials from which liquid-crystalline media are predominantlycomposed; however, compounds of the formula I can also be added toliquid-crystalline base materials from other classes of compound inorder, for example, to modify the dielectric and/or optical anisotropyof a dielectric of this type and/or to optimize its threshold voltageand/or its viscosity.

In the pure state, the compounds of the formula I are colourless andform liquid-crystalline mesophases in a temperature range which isfavourably located for electro-optical use. They are stable chemically,thermally and to light.

Compounds of the formula

where X=F, Cl, CF₃, CHF₂, OCHF₂, or OCF₃, Z=H or F and ringA=1,4-cyclohexylene or 1,4-phenylene, have already been disclosed in WO91/13850.

In the novel media comprising compounds of the formula I, Y ispreferably F, Cl, OCF₃, OCHF₂, CF₃, CHFCF₃, CF₂CHF₂, C₂H₄CHF₂,CF₂CH₂CF₃, CHF₂, OCH₂CF₃, OCH₂CHF₂, OCF₂CHF₂, O(CH₂)₃CF₃, OCH₂C₂F₅,OCH₂CF₂CHF₂, OCH₂C₃F₇, OCHFCF₃, OC₂F₅, OCF₂CHFCF₃, OCH═CF₂, OCF═CF₂,OCF═CFCF₃, OCF═CF—C₂F₅, CH═CHF, CH═CF₂, CF═CF₂, CF₂OCF₃, in particularF, OCHFCF₃, OCF₃, OCHF₂, OC₂F₅, OC₃F₇, OCH═CF₂ or CF₂OCF₃.

Particular preference is given to compounds of the formula I in whichL=F and/or m=0.

Z¹ is preferably a single bond or —CH₂CH₂—, secondarily preferably—CH₂O—, —OCH₂—, —O—CO—or —CO—O—.

If R is an alkyl radical and/or an alkoxy radical, this can bestraight-chain or branched. It is preferably straight-chain, has 2, 3,4, 5, 6 or 7 carbon atoms and accordingly is preferably ethyl, propyl,butyl, pentyl, hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxyor heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl,tridecyl, tetradecyl, pentadecyl, methoxy, octoxy, nonoxy, decoxy,undecoxy, dodecoxy, tridecoxy or tetradecoxy.

Oxaalkyl is preferably straight-chain 2-oxapropyl (=methoxymethyl),2-(=ethoxymethyl) or 3-oxabutyl (=2-methoxyethyl), 2-, 3- or4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl,2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl,or 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R is an alkyl radical in which one CH₂ group has been replaced by—CH═CH—, this can be straight-chain or branched. It is preferablystraight-chain, has 2 to 10 carbon atoms and is vinyl, IE-alkenyl or3E-alkenyl. Accordingly, it is in particular vinyl, prop-1- or -2-enyl,but-1-, -2- or -3-enyl, pent-1-, -2-, -3- or -4-enyl, hex-1-, -2-, -3-,-4- or -5-enyl, hept-1-, -2-, -3-, -4-, -5- or -6-enyl, oct-1-, -2-,-3-, -4-, -5-, -6- or -7-enyl, non-1-, -2-, -3-, -4-, -5-, -6-, -7- or-8-enyl, or dec-1-, -2-, -3-, -4-, -5-, -6-, -7-, -8- or -9-enyl.

If R is an alkyl radical in which one CH₂ group has been replaced by —O—and one has been replaced by —CO—, these are preferably adjacent. Thesethus contain one acyloxy group —CO—O— or one oxycarbonyl group —O—CO—.These are preferably straight-chain and have 2 to 6 carbon atoms.Accordingly, they are in particular acetoxy, propionyloxy, butyryloxy,pentanoyloxy, hexanoyloxy, acetoxymethyl, propionyloxymethyl,butyryloxymethyl, pentanoyloxymethyl, 2-acetoxyethyl,2-propionyloxyethyl, 2-butyryloxyethyl, 3-acetoxypropyl,3-propionyloxypropyl, 4-acetoxybutyl, methoxycarbonyl, ethoxycarbonyl,propoxycarbonyl, butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl,ethoxycarbonylmethyl, propoxycarbonylmethyl, butoxy-carbonylmethyl,2-(methoxycarbonyl)ethyl, 2-(ethoxy-carbonyl)ethyl,2-(propoxycarbonyl)ethyl, 3-(methoxy-carbonyl)propyl,3-(ethoxycarbonyl)propyl or 4-(methoxycarbonyl)butyl.

If R is an alkyl radical in which one CH₂ group has been replaced byunsubstituted or substituted —CH═CH— and an adjacent CH₂ group has beenreplaced by CO or CO—O or O—CO, this can be straight-chain or branched.It is preferably straight-chain and has 4 to 13 carbon atoms.Accordingly, it is in particular acryloyloxymethyl, 2-acryloyloxyethyl,3-acryloyloxypropyl, 4-acryloyloxybutyl, 5-acryloyloxypentyl,6-acryloyloxyhexyl, 7-acryloyloxyheptyl, 8-acryloyloxyoctyl,9-acryloyloxynonyl, 10-acryloyloxydecyl, methacryloyloxymethyl,2-methacryloyloxyethyl, 3-methacryloyloxypropyl, 4-methacryloyloxybutyl,5-methacryloyloxypentyl, 6-methacryloyloxyhexyl,7-methacryloyloxyheptyl, 8-methacryloyloxyoctyl and9-methacryloyloxynonyl.

If R is an alkyl or alkenyl radical which is monosubstituted by CN orCF₃, this radical is preferably straight-chain. The substitution by CNor CF₃ is in any desired position.

If R is an alkyl or alkenyl radical which is at least monosubstituted byhalogen, this radical is preferably straight-chain and halogen ispreferably F or Cl. In the case of multiple substitution, halogen ispreferably F. The resultant radicals also include perfluorinatedradicals. In the case of monosubstitution, the fluorine or chlorinesubstituent can be in any desired position, but preferably in theω-position.

Compounds of the formula I which contain wing groups R which aresuitable for polyaddition reactions are suitable for the preparation ofliquid-crystalline polyaddition products.

Compounds of the formula I containing branched wing groups R mayoccasionally be of importance due to better solubility in the customaryliquid-crystalline base materials, but in particular as chiral dopes ifthey are optically active. Smectic compounds of this type are suitableas components for ferroelectric materials.

Compounds of the formula I having S_(A) phases are suitable, forexample, for thermally addressed displays.

Branched groups of this type generally contain not more than one chainbranch. Preferred branched radicals R are isopropyl, 2-butyl(=1-methylpropyl), isobutyl (=2-methylpropyl), 2-methylbutyl, isopentyl(=3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl,2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy,3-methylbutoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy,1-methylhexoxy and 1-methylheptoxy.

If R is an alkyl radical in which two or more CH₂ groups have beenreplaced by —O— and/or —CO—O—, this may be straight-chain or branched.It is preferably branched and has 3 to 12 carbon atoms. Accordingly, itis in particular biscarboxymethyl, 2,2-biscarboxyethyl,3,3-biscarboxypropyl, 4,4-biscarboxybutyl, 5,5-biscarboxypentyl,6,6-biscarboxyhexyl, 7,7-biscarboxyheptyl, 8,8-biscarboxyoctyl,9,9-biscarboxynonyl, 10,10-biscarboxydecyl, bis(methoxycarbonyl)methyl,2,2-bis(methoxycarbonyl)ethyl, 3,3-bis(methoxycarbonyl)propyl,4,4-bis(methoxycarbonyl)butyl, 5,5-bis(methoxycarbonyl)pentyl,6,6-bis(methoxycarbonyl)hexyl, 7,7-bis(methoxycarbonyl)heptyl,8,8-bis(methoxycarbonyl)-octyl, bis(ethoxycarbonyl)methyl,2,2-bis(ethoxycarbonyl)ethyl, 3,3-bis(ethoxycarbonyl)propyl,4,4-bis(ethoxycarbonyl)butyl and 5,5-bis(ethoxycarbonyl)-hexyl.

Preferred smaller groups of compounds of the formula I are those of thesubformulae I1 to I5 [L=H or F]:

Particular preferrence is given to the compounds of the formulae I1 andI2.

The 1,4-cyclohexenylene group preferably has the following structures:

The compounds of the formula I are prepared by methods known per se, asdescribed in the literature (for example in the standard works, such asHouben-Weyl, Methoden der Organischen Chemie, Georg-Thieme-Verlag,Stuttgart) to be precise under reaction conditions which are known andsuitable for said reactions. Use can also be made here of variants whichare known per se.

The compounds according to the invention can be prepared, for example,by metallating a compound of formula IA

in which R, A¹, Z¹, L and m are as defined above, and subsequentlyreacting the product with a suitable electrophile, or by a couplingreaction as follows:

The invention also relates to electro-optical displays (in particularSTN or MLC displays having two plane-parallel outer plates which,together with a frame, form a cell, integrated nonlinear elements forswitching individual pixels on the outer plates, and a nematicliquid-crystal mixture of positive dielectric anisotropy and highresistivity located in the cell) which contain media of this type, andto the use of these media for electro-optical purposes.

The liquid-crystal mixtures according to the invention facilitate asignificant broadening of the parameter latitude available.

The achievable combinations of rotational viscosity γ₁, clearing point,viscosity at low temperature, thermal and UV stability and dielectricanisotropy are far superior to previous materials from the prior art.

The requirement for a high clearing point, a nematic phase at lowtemperature and a high Δ∈ was previously only achievable to anunsatisfactory extent. Although systems such as, for example, ZLI-3119have a comparable clearing point and comparatively favourableviscosities, they have, however, a Δ∈ of only +3.

Other mixture systems have comparable viscosities and values of Δ∈, butonly have clearing points in the region of 60° C.

The liquid-crystal mixtures according to the invention make it possibleto achieve clearing points of above 80°, preferably above 90°,particularly preferably above 100° C., and simultaneously dielectricanisotropy values Δ∈≧6, preferably ≧8, and a high value for theresistivity while retaining the nematic phase down to −20° C. andpreferably down to −30° C., particularly preferably down to −40° C.,which allows excellent STN and MLC displays to be achieved. Inparticular, the mixtures are characterized by low operating voltages.The TN thresholds are below 2.0 V, preferably below 1.6 V, particularlypreferably <1.3 V.

It goes without saying that a suitable choice of the components of themixtures according to the invention also allows higher clearing points(for example above 110°) to be achieved at higher threshold voltages orlower clearing points to be achieved at lower threshold voltages whileretaining the other advantageous properties. It is likewise possible toobtain mixtures of relatively high Δ∈ and thus relatively low thresholdsif the viscosities are increased by a correspondingly small amount. TheMLC displays according to the invention preferably operate in the firsttransmission minimum of Gooch and Tarry [C. H. Gooch and H. A. Tarry,Electron. Lett. 10, 2–4, 1974; C. H. Gooch and H. A. Tarry, Appl. Phys.,Vol. 8, 1575–1584, 1975]; in this case, a lower dielectric anisotropy inthe second minimum is sufficient in addition to particularly favourableelectro-optical properties, such as, for example, high gradient of thecharacteristic line and low angle dependency of the contrast (GermanPatent 30 22 818, U.S. Pat. No. 4,398,803) at the same time thresholdvoltage as in an analogous display. This allows significantly higherresistivities to be achieved in the first minimum using the mixturesaccording to the invention than using mixtures containing cyanocompounds. A person skilled in the art can use simple routine methods toproduce the birefringence necessary for a prespecified layer thicknessof the MLC display by a suitable choice of the individual components andtheir proportions by weight.

The flow viscosity at 20° C. is preferably <60 mm².s⁻¹, particularlypreferably <50 mm².s⁻¹. The nematic phase range is preferably at least90°, in particular at least 100°. This range preferably extends at leastfrom −20° to +80°.

Measurements of the “capacity holding ratio” (HR) [S. Matsumoto et al.,Liquid Crystals 5, 1320 (1989); K. Niwa et al., Proc. SID Conference,San Francisco, June 1984, p. 304 (1984); G. Weber et al., LiquidCrystals 5, 1381 (1989)] have shown that mixtures according to theinvention comprising compounds of the formula I exhibit a considerablysmaller decrease in the HR with increasing temperature than do analogousmixtures in which the compounds of the formula I are replaced bycyanophenylcyclohexanes of the formula

or esters of the formula

The liquid crystal mixtures preferably contain less than 50 wt. %, morepreferably less than 35 wt. % and particularly preferably less than 15%wt. of cyano compounds.

The UV stability of the mixtures according to the invention is alsoconsiderably better, i.e. they exhibit a significantly smaller decreasein the HR on exposure to UV radiation.

The media according to the invention are preferably based on a plurality(preferably two or more) of compounds of the formula I, i.e. theproportion of these compounds is 5–95%, preferably 10–60% andparticularly preferably in the range 15–50%.

The individual compounds of the formulae I to XVI and their sub-formulaewhich can be used in the media according to the invention are eitherknown or can be prepared analogously to the known compounds.

Preferred embodiments are indicated below:

-   -   Medium comprises compounds of the formula I in which R is        preferably ethyl, furthermore propyl, butyl or pentyl. Compounds        of the formula I having short side chains R have a positive        effect on the elastic constants, in particular K₁, and give        mixtures having particularly low threshold voltages.    -   Medium additionally comprises one or more compounds selected        from the group consisting of the general formulae II to IX:

-   -   in which the individual radicals have the following meanings:    -   R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case        having up to 9 carbon atoms,    -   X⁰ is F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6        carbon atoms,    -   Y¹ to Y⁴ are in each case, independently of one another, H or F,    -   r is 0 or 1.

The compound of the formula IV is preferably

-   -   Medium additionally comprises one or more compounds of the        formula

-   -   Medium additionally comprises one or more compounds of the        formulae RI and/or RII:

-   -    in which R⁰ is as defined above, preferably straight-chain        alkyl having 1–6 carbon atoms, and alkenyl and alkenyl are        preferably each, independently of one another, vinyl,        1E-alkenyl, 3E-alkenyl or 4-alkenyl having up to 9 carbon atoms.    -   Medium additionally comprises one or more compounds selected        from the group consisting of the general formulae X to XVI:

in which

-   R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having    up to 9 carbon atoms,-   X⁰ is F, Cl, halogenated alkyl, alkenyl or alkoxy having 1 to 6    carbon atoms,-   Y¹ and Y² are in each case, independently of one another H or F.

Preferably, R⁰, X⁰, Y¹ and Y² are F, Cl, CF₃, OCHF₂, alkyl, oxaalkyl,fluro-alkyl or alkenyl, in each case having up to 6 carbon atoms, whereconsistent with the above definitions.

-   -   The proportion of compounds of the formulae I to IX together is        at least 50% by weight in the total mixture;    -   The proportion of compounds of the formula I is from 10 to 50%        by weight in the total mixture;    -   The proportion of compounds of the formulae II to IX is from 20        to 80% by weight in the total mixture

is preferably

-   -   The medium comprises compounds of the formulae II, III, IV, V,        VI, VII, VIII and/or IX    -   R⁰ is straight-chain alkyl or alkenyl having 2 to 7 carbon atoms    -   The medium essentially consists of compounds of the formulae I        to IX    -   The medium comprises further compounds, preferably selected from        the following group consisting of the general formulae XVII to        XXII:

-   -    in which R⁰, X⁰ and X^(0′) are as defined above, and the        1,4-phenylene rings may be substituted by CN, chlorine or        fluorine.

The 1,4-phenylene rings are preferably monosubstituted orpolysubstituted by fluorine atoms.

-   -   The I: (II+III+IV+V+VI+VII+VIII+IX) weight ratio is preferably        from 1:10 to 10:1    -   Medium essentially consists of compounds selected from the group        consisting of the general formulae I to XXII.

It has been found that even a relatively small proportion of compoundsof the formula I mixed with conventional liquid-crystal materials, butin particular with one or more compounds of the formulae II to IX,results in a significant reduction in the rational viscosity and inlow-birefringence values, and at the same time broad nematic phases withlow smectic-nematic transition temperatures are observed, thus improvingthe shelf life. The compounds of the formulae I to IX are colourless,stable and readily miscible with one another and with otherliquid-crystal materials.

The term “alkyl”, covers straight-chain and branched alkyl groups having1–7 carbon atoms, in particular the straight-chain groups methyl, ethyl,propyl, butyl, pentyl, hexyl and heptyl. Groups having 2–5 carbon atomsare generally preferred.

The term “alkenyl” or “alkenyl*” covers straight-chain and branchedalkenyl groups having 2–7 carbon atoms, in particular the straight-chaingroups. Particular alkenyl groups are C₂–C₇-1E-alkenyl,C₄–C₇-3E-alkenyl, C₅–C₇-4-alkenyl, C₆–C₇-5-alkenyl and C₇-6-alkenyl, inparticular C₂–C₇-1E-alkenyl, C₄–C₇-3E-alkenyl and C₅–C₇-4-alkenyl.Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl,1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl,3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl,4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5carbon atoms are generally preferred.

The term “fluoroalkyl” preferably covers straight-chain groupscontaining terminal fluorine, i.e. fluoromethyl, 2-fluoroethyl,3-fluoropropyl, 4-fluorobutyl, 5-fluoropentyl, 6-fluorohexyl and7-fluoroheptyl. However, other positions of the fluorine are notexcluded.

The term “oxaalkyl” preferably covers straight-chain radicals of theformula C_(n)H_(2n+1)—O—(CH₂)_(m), in which n and m are each,independently of one another, from 1 to 6, n is preferably 1 and m ispreferably from 1 to 6.

Through a suitable choice of the meanings of R⁰ and X⁰, the addressingtimes, the threshold voltage, the gradient of the transmissioncharacteristic lines, etc., can be modified as desired. For example,1E-alkenyl radicals, 3E-alkenyl radicals, 2E-alkenyloxy radicals and thelike generally give shorter addressing times, improved nematictendencies and a higher ratio between the elastic constants k₃₃ (bend)and k₁₁ (splay) compared with alkyl and alkoxy radicals. 4-Alkenylradicals, 3-alkenyl radicals and the like generally give lower thresholdvoltages and lower values of k₃₃/k₁₁ compared with alkyl and alkoxyradicals.

A —CH₂CH₂— group in Z¹ generally results in higher values of k₃₃/k₁₁compared with a simple covalent bond. Higher values of k₃₃/k₁₁facilitate, for example, flatter transmission characteristic lines in TNcells with a 90° twist (for achieving grey tones) and steepertransmission characteristic lines in STN, SBE and OMI cells (greatermultiplexibility), and vice versa.

The optimum mixing ratio of the compounds of the formulae I andII+III+IV+V+VI+VII+VIII+IX depends substantially on the desiredproperties, on the choice of the components of the formulae I, II, III,IV, V, VI, VII, VIII and/or IX and on the choice of any other componentswhich may be present. Suitable mixing ratios within the abovementionedrange can easily be determined from case to case.

The total amount of compounds of the formulae I to XVI in the mixturesaccording to the invention is not crucial. The mixtures may thereforecontain one or more further components in order to optimize variousproperties. However, the effect observed on the addressing times and thethreshold voltage is generally greater the higher the totalconcentration of compounds of the formulae I to XVI.

In a particularly preferred embodiment, the media according to theinvention comprise compounds of the formulae II to IX (preferably IIand/or III) in which X⁰ is OCF₃, OCHF₂, F, OCH═CF₂, OCF═CF₂ orOCF₂—CF₂H. A favourable synergistic effect with the compounds of theformula I results in particularly advantageous properties.

The construction of the MLC display according to the invention frompolarizers, electrode base plates and electrodes with surface treatmentcorresponds to the construction which is conventional for displays ofthis type. The term conventional construction here is broadly drawn andalso covers all derivatives and modifications of the MLC display, inparticular also matrix display elements based on poly-Si TFTs or MIMs.

An essential difference between the displays according to the inventionand those customary hitherto based on the twisted nematic cell is,however, the choice of the liquid-crystal parameters in theliquid-crystal layer.

The liquid-crystal mixtures which can be used according to the inventionare prepared in a manner which is conventional per se. In general, thedesired amount of the components used in the lesser amount is dissolvedin the components making up the principal constituent, expediently atelevated temperature. It is also possible to mix solutions of thecomponents in an organic solvent, for example in acetone, chloroform ormethanol, and, after thorough mixing, to remove the solvent again, forexample by distillation.

The dielectrics may also contain other additives known to those skilledin the art and described in the literature. For example, 0–15% ofpleochroic dyes or chiral dopes can be added.

C denotes a crystalline phase, S a smectic phase, S_(C) a smectic Cphase, N a nematic phase and I the isotropic phase.

V₁₀ denotes the voltage for 10% transmission (view angle perpendicularto the plate surface) t_(on) denotes the switch-on time and t_(off) theswitch-off time at an operating voltage corresponding to 2.5 times thevalue of V₁₀. Δn denotes the optical anisotropy and no the refractiveindex. Δ∈ denotes the dielectric anisotropy (Δ∈=∈_(∥−∈) _(⊥), where∈_(∥) is the dielectric constant parallel to the longitudinal molecularaxes and ∈_(⊥) is the dielectric constant perpendicular thereto). Theelectro-optical data were measured in a TN cell at the 1st minimum (i.e.at a d·Δn value of 0.5) at 20° C., unless expressly stated otherwise.The optical data were measured at 20° C., unless expressly statedotherwise.

In the present application and in the examples below, the structures ofthe liquid-crystal compounds are indicated by acronyms, with thetransformation into chemical formulae taking place in accordance withTables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) arestraight-chain alkyl radicals containing n or m carbon atomsrespectively. The coding in Table B is self-evident. In Table A, onlythe acronym for the base structure is given. In individual cases, theacronym for the base structure is followed, separated by a hyphen, by acode for the substituents R¹, R², L¹ and L²:

Code for R¹, R², L¹, L² R¹ R² L¹ L² nm C_(n)H_(2n+1) C_(m)H_(2m+1) H HnOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) HH n C_(n)H_(2n+1) CN H H nN.F C_(n)H_(2n+1) CN H F nF C_(n)H_(2n+1) F HH nOF OC_(n)H_(2n+1) F H H nCl C_(n)H_(2n+1) Cl H H nF.F C_(n)H_(2n+1) FH F nF.F.F C_(n)H_(2n+1) F F F nCF₃ C_(n)H_(2n+1) CF₃ H H nOCF₃C_(n)H_(2n+1) OCF₃ H H nOCF₂ C_(n)H_(2n+1) OCHF₂ H H nS C_(n)H_(2n+1)NCS H H rVsN C_(r)H_(2r+1)—CH═ CN H H CH—C_(s)H_(2s)— rEsNC_(r)H_(2r+1)—O—C₂H_(2s)— CN H H nAm C_(n)H_(2n+1) COOC_(m)H_(2m+1) H HnOCCF₂.F.F C_(n)H_(2n+1) OCH₂CF₂H F F

Preferred mixture components are shown in Tables A and B.

TABLE A

TABLE B

The examples below are intended to illustrate the invention withoutrepresenting a limitation. Above and below, percentages are percent byweight. All temperatures are given in degrees Celsius. m.p. denotesmelting point, c.p.=clearing point. Furthermore, C=crystalline state,N=nematic phase, S=smectic phase and I=isotropic phase. The data betweenthese symbols represent the transition temperatures. Δn denotes theoptical anisotropy (589 nm, 20° C.), and the flow viscosity (mm²/sec)was determined at 20° C.

MIXTURE EXAMPLES Example 1

ECCP-3F.F 7.0% S → N [° C.]: >−30 ECCP-5F.F 10.0%  Clearing point [°C.]: +106.5° C. BCH-3F.F.F 11.0%  Δn [589 nm, 20° C.]: +0.1468BCH-5F.F.F 12.0%  V_((10,0,20)) [V]: 1.54 BCH-32 5.0% γ₁ [mPa · s]: 242BCH-52 3.0% BCH-2F.F 9.0% BCH-3F.F 9.0% BCH-5F.F 9.0% PGU-3-F 17.0% CBC-33 3.0% CBC-53 3.0% CBC-55 2.0%

Example 2

CGU-2-F 10.0% S → N [° C.]: <−40.0 CGU-3-F 10.0% Clearing point [° C.]:+69.5° C. CGU-5-F  4.0% Δn [589 nm, 20° C.]: +0.1110 PGU-2-F  8.0% Δε [1kHz, 20° C.]: 12.0 PGU-3-F 10.0% K₃/K₁ [20° C.] 1.28 CCP-30CF₃  8.0% γ₁[20° C.] [mPa · s]: 171 CCP-50CF₃  8.0% V_((10,0,20)) [V]: 1.10CCP-2F.F.F 12.0% CCP-3F.F.F 10.0% CCP-20CF₃.F 10.0% CCP-30CF₃.F  7.0%CBC-33F  3.0%

Example 3

CGH-34 5.0% S → N [° C.]: <−20.0 CGU-2-F 6.0% Clearing point [° C.]:+67.5° C. CGU-3-F 9.0% Δn [589 nm, 20° C.]: +0.1131 CGU-5-F 7.0%V_((10,0,20)) [V]: 1.09 CCP-20CF₃.F 8.0% CCP-30CF₃.F 11.0%  CCP-2F.F.F11.0%  CCP-3F.F.F 10.0%  CCP-30CF₃ 8.0% PGU-2-F 10.0%  PGU-3-F 12.0% CBC-33F 3.0%

Example 4

CGU-2-F 9.0% S → N [° C.]: <−40.0 CGU-3-F 9.0% Clearing point [° C.]:+72.5° C. CGU-5-F 2.0% Δn [589 nm, 20° C.]: +0.1220 CCP-30CF₃.F 11.0% Δε [1 kHz, 20° C.]: +13.1 CCP-2F.F.F 12.0%  γ₁ [20° C.] [mPa · s]: 165CCP-3F.F.F 10.0%  V_((10,0,20)) [V]: 1.07 CCP-30CF₃ 8.0% CCP-50CF₃ 7.0%PGU-2-F 10.0%  PGU-3-F 10.0%  PGU-5-F 7.0% CBC-33F 5.0%

Example 5

CGU-2-F  8.0% S → N [° C.]: <−30.0 CGU-3-F 10.0% Clearing point [° C.]:+69.0° C. CGU-5-F 10.0% Δn [589 nm, 20° C.]: +0.1415 BCH-5F.F.F 11.0% Δε[1 kHz, 20° C.]: +14.0 BCH-5F.F  8.0% γ₁ [20° C.] [mPa · s]: 184CCP-30CF₃  8.0% V_((10,0,20)) [V]: 1.02 CCP-50CF₃  7.0% PGU-2-F 10.0%PGU-3-F 10.0% PGU-5-F 12.0% CBC-33F  4.0% CBC-53F  2.0%

Example 6

CGU-2-F 6.0% S → N [° C.]: <−30.0 CGU-3-F 10.0%  Clearing point [° C.]:+69.5° C. CGU-5-F 9.0% Δn [589 nm, 20° C.]: +0.1209 BCH-3F.F.F 7.0% Δε[1 kHz, 20° C.]: +15.1 CCP-30CF₃.F 6.0% γ₁ [20° C.] [mPa · s]: 185CCP-3F.F.F 9.0% V_((10,0,20)) [V]: 0.98 CCP-30CF₃ 8.0% PGU-2-F 10.0% PGU-3-F 12.0%  CCZU-2-F 6.0% CCZU-3-F 14.0%  CBC-33 3.0%

Example 7

CGU-2-F 11.0%  S → N [° C.]: <−40.0 CGU-3-F 9.0% Clearing point [° C.]:+70.5° C. CGU-5-F 0.0% Δn [589 nm, 20° C.]: +0.1209 CCP-2F.F.F 11.0%  Δε[1 kHz, 20° C.]: +14.9 CCP-3F.F.F 4.0% γ₁ [20° C.] [mPa · s]: 162CCZU-3-F 15.0%  V_((10,0,20)) [V]: 0.95 CCZU-5-F 6.0% CCP-30CF₃ 7.0%CCP-50CF₃ 4.0% PGU-2-F 10.0%  PGU-3-F 10.0%  PGU-5-F 8.0% CBC-33F 5.0%

Example 8

GGP-5-Cl 14.0%  Clearing point [° C.]: 111.0° C. T-3FCIF 10.0%  Δn [589nm, 20° C.]: +0.2076 PGU-2-F 5.0% V_((10,0,20)) [V]: 1.92 PGU-3-F 8.0%PGU-5-F 6.0% FET-2Cl 10.0%  FET-3Cl 8.0% CGU-3-F 8.0% BCH-3F.F 3.0%BCH-5F.F 12.0%  CCGU-3-F 7.0% CBC-33 3.0% CBC-53 3.0% CBC-55 3.0%

Example 9

GGP-5-Cl 15.00%  Clearing point [° C.]: +111.0° C. T-3FClF 12.00%  Δn[589 nm, 20° C.]: +0.2081 T-5FClF 4.00% V_((10,0,20)) [V]: 2.00 PGU-2-F6.00% PGU-3-F 10.00%  PGU-5-F 13.00%  BCH-2F.F 12.00%  BCH-5F.F.F12.00%  CCGU-3-F 7.00% CBC-33 3.00% CBC-53 3.00% CBC-55 3.00%

Example 10

GGP-5-Cl 14.00%  Clearing point [° C.]: +110.0° C. T-3FClF 10.00%  Δn[589 nm, 20° C.]: +0.2066 PGU-2-F 4.00% V_((10,0,20)) [V]: 1.87 PGU-3-F8.00% PGU-5-F 12.00%  FET-2Cl 5.00% FET-3Cl 8.00% CGU-3-F 5.00% BCH-2F.F7.00% BCH-3F.F 6.00% BCH-5F.F 6.00% CCGU-3-F 6.00% CBC-33 3.00% CBC-533.00% CBC-55 3.00%

Example 11

GGP-3-Cl 13.00% Clearing point [° C.]: +104.0° C. GGP-5-Cl 13.00% Δn[589 nm, 20° C.]: +0.2145 T-3FClF  8.00% V_((10,0,20)) [V]: 2.14 FET-2Cl12.00% FET-3Cl  8.00% FET-5Cl 13.00% BCH-2F.F 12.00% BCH-5F.F.F 10.00%CCGU-3-F  7.00% CBC-33  2.00% CBC-53  2.00%

Example 12

GGP-3-Cl 12.00%  Clearing point [° C.]: +109.0° C. GGP-5-Cl 12.00%  Δn[589 nm, 20° C.]: +0.2143 T-3FClF 8.00% V_((10,0,20)) [V]: 1.88 PGU-2-F6.00% PGU-3-F 8.00% FET-2Cl 12.00%  FET-3Cl 6.00% CGU-3-F 9.00% BCH-3F.F6.00% BCH-5F.F 8.00% CCGU-3-F 6.00% CBC-33 3.00% CBC-53 3.00% CBC-551.00%

Example 13

GGP-5-Cl 14.00%  Clearing point [° C.]: +111.0° C. T-3FClF 8.00% Δn [589nm, 20° C.]: +0.2074 PGU-2-F 6.00% V_((10,0,20)) [V]: 1.86 PGU-3-F10.00%  PGU-5-F 13.00%  FET-2Cl 4.00% FET-3Cl 8.00% CGU-5-F 5.00%BCH-3F.F 4.00% BCH-5F.F 12.00%  CCGU-3-F 7.00% CBC-33 3.00% CBC-53 3.00%CBC-55 3.00%

Example 14

GGP-5-Cl 12.00%  Clearing point [° C.]: < +112.0° C. T-3FClF 10.00%  Δn[589 nm, 20° C.]: +0.2118 PGU-2-F 6.00% V_((10,0,20)) [V]: 2.06 PGU-3-F11.00%  PGU-5-F 13.00%  FET-2Cl 5.00% FET-3Cl 8.00% CCGU-3-F 6.00%BCH-3F.F 9.00% BCH-5F.F 11.00%  CBC-33 3.00% CBC-53 3.00% CBC-55 3.00%

The entire disclosure of all applications, patents and publications,cited above, and of corresponding applications German PatentApplications DE 19819392.0 and DE 19843582.7 are hereby incorporated byreference.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactants and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A liquid-crystalline medium comprising a mixture of polar compoundshaving positive dielectric anisotropy, wherein said mixture comprisesone or more compounds of formula I,

in which R is H, an alkyl or alkenyl radical having 1 to 15 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, it also being possible for one or more CH₂groups in these radicals to be replaced, in each case independently ofone another by —O—

—CO—, —CO—O—, —O—CO—, —S— or —O—CO—O— in such a way that O atoms are notlinked directly to one another, A¹ (a) is a trans-1,4-cyclohexyleneradical in which one CH₂ group is replaced by —O— or —S—, where theradical (a) may be monosubstituted or polysubstituted by CN, CH₃ or F,Z¹ is —CO—O—, —O—CO—, —CH₂—O—, —OCH₂—, —CH₂CH₂—, —CF₂O—, —OCF₂—,—CH═CH—, —C≡C—, —(CH₂)₄—, —CH═CH—CH₂CH₂— or a single bond, Y is F, Cl,halogenated alkyl having 1 to 6 carbon atoms, halogenated alkenyl havingup to 6 carbon atoms, or halogenated alkoxy having 1 to 6 carbon atoms,L is H or F, and m is
 1. 2. A medium according to claim 1, wherein saidmixture additionally comprises one or more compounds of formulae II toIX:

in which the individual radicals have the following meanings: R⁰ isn-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each case having up to 9carbon atoms, X⁰ is F, Cl, halogenated alkyl having 1 to 6 carbon atoms,halogenated alkenyl having up to 6 carbon atoms, or halogenated alkoxyhaving 1 to 6 carbon atoms, Y¹ to Y⁴ are in each case, independently ofone another, H or F, and r is 0 or
 1. 3. A medium according to claim 2,wherein the proportion of compounds of the formulae I to IX together isat least 50% by weight in the total mixture.
 4. A medium according toclaim 1, wherein the proportion of compounds of the formula I is from 10to 50% by weight in the total mixture.
 5. A medium according to claim 2,wherein the proportion of compounds of the formulae II to IX is from 20to 80% by weight in the total mixture.
 6. A medium according to claim 1,wherein said mixture further comprises a compound of formula 12

wherein R is H, an alkyl or alkenyl radical having 1 to 15 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, it also being possible for one or more CH₂groups in these radicals to be replaced, in each case independently ofone another by —O—,

—CO—, —CO—O—, —O—CO—, —S— or —O—CO—O— in  such a way that O atoms arenot linked directly to one another, and Y is F, Cl, halogenated alkylhaving 1 to 6 carbon atoms, halogenated alkenyl having up to 6 carbonatoms or halogenated alkoxy having 1 to 6 carbon atoms.
 7. A mediumaccording to claim 6, wherein in formula 12 Y is F, OCHF₂ or OCF₃.
 8. Amedium according to claim 1, wherein said mixture additionally comprisesone or more compounds of the formula

in which

R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9carbon atoms, and L¹ is H or F.
 9. A medium according to claim 1,wherein said mixture additionally comprises one or more compounds offormula XI

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each casehaving up to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl having 1 to6 carbon atoms, halogenated alkenyl having up to 6 carbon atoms orhalogenated alkoxy having 1 to 6 carbon atoms, and Y¹ and Y² are in eachcase, independently of one another, H or F.
 10. In an electroopticaldisplay containing a liquid crystal medium, the improvement wherein saidmedium is a medium according to claim
 1. 11. In a method of generatingan electrooptical effect using a liquid crystal display, the improvementwherein said display is a display according to claim
 10. 12. A mediumaccording to claim 3, wherein the proportion of compounds of theformulae II to IX is from 20 to 80% by weight in the total mixture. 13.A medium according to claim 2, wherein said mixture further comprises acompound of formula 12

wherein R is H, an alkyl or alkenyl radical having 1 to 15 carbon atomswhich is unsubstituted, monosubstituted by CN or CF₃ or at leastmonosubstituted by halogen, it also being possible for one or more CH₂groups in these radicals to be replaced, in each case independently ofone another by —O—,

—CO—, —CO—O—, —O—CO—, —S— or —O—CO—O— in such a way that O atoms are notlinked directly to one another, and Y is F, Cl, halogenated alkyl,alkenyl or alkoxy having 1 to 6 carbon atoms.
 14. A medium according toclaim 13, wherein in formula 12 Y is F, OCHF₂ or OCF₃.
 15. A mediumaccording to claim 6, wherein said mixture additionally comprises one ormore compounds of the formula

in which

R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, each having up to 9carbon atoms, and is H or F.
 16. A medium according to claim 6, whereinsaid mixture additionally comprises one or more compounds of formula XI

in which R⁰ is n-alkyl, oxaalkyl, fluoroalkyl or alkenyl, in each casehaving up to 9 carbon atoms, X⁰ is F, Cl, halogenated alkyl, alkenyl oralkoxy having 1 to 6 carbon atoms, and Y¹ and Y² are in each case,independently of one another, H or F.
 17. In an electrooptical displaycontaining a liquid crystal medium, the improvement wherein said mediumis a medium according to claim
 6. 18. In a method of generating anelectrooptical effect using a liquid crystal display, the improvementwherein said display is a display according to claim
 17. 19. A mediumaccording to claim 1, wherein Y is F, OCHFCF₃, OCF₃, OCHF₂, OC₂F₅,OC₃F₇, OCH═CF₂, or CF₂OCF₃.